Faster than light communication

[Ian75]

Imagine a small cylinder with a dot at the top. Rotating the cylinder 1 degree clockwise of 0 degrees could signify a 1; and 1 degree anticlockwise of 0 degrees a zero. 0 degrees would be a space.

Someone rotating one end of the cylinder would cause the other end of the cylinder to rotate instantly. Now imagine the cylinder extended to the length of one light year. Rotating one end of the cylinder would cause the other end to rotate instantly, yet a signal sent from the same end and traveling at the speed of light would take one year to arrive. This would be faster than light communication - or am I missing something really obvious?

 

[fresh_42]

This would be faster than light communication - or am I missing something really obvious?

Yes. This (flagged by me):

Rotating one end of the cylinder would cause the other end to rotate instantly

Instantly is wrong. It takes time to transport the force from one end to the other end and this is even slower than the speed of light. The cylinder is only kept in one piece by electromagnetic forces on molecule level. So an action on one end has to be transported via this chain to the other end.

 

[pixel]

Nothing is perfectly rigid.

 

[rumborak]

This is very similar to the thought experiment of taking a flashlight and sweeping it. Far enough away, the argument goes, the light beam would sweep with superluminal speed. But, what actually happens is that the beam of light behaves more like a waterhose at that point.

 

[Dale]

Someone rotating one end of the cylinder would cause the other end of the cylinder to rotate instantly.

Think of a cylinder made of jello. If you rotate one end then you will set up a torsion wave that moves so slow that you can see it.

Steel isn't perfectly rigid, and on relativistic it would be like jello.

 

[Nugatory]

This question come up often enough that we have a FAQ on it: https://www.physicsforums.com/insights/can-i-send-a-signal-faster-than-light-by-pushing-a-rigid-rod/

That's for pushing one end of a "rigid" rod. but twisting an end of the rod is basically the same thing.

 

[Drakkith]

This thread is attracting FTL posts which are violations of PF rules, so I'm going to go ahead and close it.

Thread locked.